This invention relates to a light-emitting diode circuit wherein a coil is connected to a voltage source, the voltage of which is lower than the threshold voltage of the forward diode characteristic.
Light-emitting diodes are preferably supplied with a restricted current. Usually, such diodes are operated from a source of voltage whose voltage is higher than the threshold voltage of the forward diode characteristic, i.e. the voltage at the knee (break point) of the current/voltage characteristic of the light-emitting diode. In the case of a gallium-arsenide-phosphide type of light-emitting diode, this voltage, as is well known, may amount to more than 1.6 V.
The German Published Patent Application (DOS) 2,255,822 discloses a circuit which is suitable for supplying a light-emitting diode also from a source of lower battery voltage with a restricted pulsating current utilizing the kick-back pulse of a coil connected thereto. In this case the light-emitting diode is connected in parallel with the coil. It is considered a disadvantage of this circuit, however, that the light-emitting diode is switched from the reverse into the forward direction with a relatively large voltage variation appearing at it, at the coil, and at the collector-emitter path of the switching transistor. This variation, in the case of a battery voltage of 1.35 V (mercury cell) amounts to about 3 V in cases where the forward threshold voltage of the light-emitting diode amounts to 1.65 V. The parasitic capacitances of the light-emitting diode, of the coil, and of the transistor must be charged and discharged by the amount of this voltage variation during each period of the switching frequency which, especially in the case of a high switching frequency, leads to additional losses. Moreover, minimum requirements must be placed on the reverse behavior of the light-emitting diode. Furthermore, it may be disadvantageous, under certain circumstances, for the light-emitting diode to be not connected on one side to reference potential.
The fact that the collector voltage of the switching transistor exceeds the battery voltage by the amount of the diode's forward threshold voltage (in the present case 1.65 V) may cause additional difficulties or at least an increased investment in cases where the switching transistor forms part of a monolithic integrated circuit with integrated devices being connected to its collector, and with the insulating islands thereof having to be connected to the positive pole of the source of supply voltage.